The present invention generally relates to a recording and/or reproduction system and more particularly, to a recording and/or reproduction system for recording signals on a recording medium or reproducing signals recorded on the recording medium.
Conventionally, for the recording and reproduction apparatus of the kind as referred to above, there have been proposed the magnetic type recording and reproduction apparatus, optical-magnetic type recording and reproduction apparatus, optical type recording and reproduction apparatus, optical or capacity type reproduction apparatus, etc., in which the optical type recording and reproduction apparatus will be taken up hereinbelow for explaining the prior art.
As one example of the optical system recording and reproduction apparatus, there may be available an arrangement so arranged that a light beam emitted from a light source such as a semi-conductor laser, etc. is converged to have a very small beam diameter by a conveying lens so as to be projected onto a rotating disc-like recording medium, whereby during recording, the light amount of the light beam is altered according to the signal to be recorded for the recording, while during reproduction, the light beam is adapted to have a weak and constant light amount so as to reproduce the signal through detection of light reflected by or transmitted through the recording medium.
The recording medium to be employed for the apparatus as described so far is, for example, so constructed that a substrate is preliminarily formed, on its surface, with a recording track in a concave and convex configuration, on the surface of which a recording material layer is formed by such means as evaporation or the like. The recording track as described above has a spiral or concentric form, with very small track pitch or interval and track width, and for example, the track interval is about 1.6 .mu.m, while the track width is 0.6 .mu.m or thereabout.
Accordingly, in the optical type recording and reproducing apparatus as described above, it is indispensable to provide a focus control means for controlling the light beam irradiated onto the rotating recording medium, to have a very small beam diameter constant at all times, and a tracking control means for controlling the light beam converged onto the recording medium, to correctly scan on the track at all times for obtaining reproduction signals at high qualities.
The focus control is effected in such a manner that the state of converging of the light beam on the recording medium is detected based on the light beam reflected from the recording medium, and the converging lens is displaced in a direction perpendicular to the surface of the recording medium according to the signal thus detected. For this focus control, an extremely high accuracy, for example, under .+-.0.5 .mu.m is required.
Meanwhile, the tracking control is so effected that by the light beam transmitted through the recording medium or reflected therefrom, positional deviation between the light beam converged on the recording medium and the track is detected, and according to the signal thus detected, the converging lens is displaced in a radial direction within the surface of the recording medium. In the above tracking control also, an extremely high accuracy, for example, under .+-.0.1 .mu.m is required in the similar manner as in the focus control.
The range in which the converging lens can be displaced in the radial direction within the recording medium surface is small at about 200 .mu.m or so at the most, and generally, in the tracking control, a transfer control is effected in which the converging lens is displaced for the light beam to follow with respect to eccentricity of the track, and the whole optical system including the light source is displaced in the radial direction of the recording medium so that the displacement of the converging lens becomes zero on the average, i.e., the converging lens moves with respect to the natural state as a center.
The known optical type recording and reproducing apparatus as described so far is required to be controlled at a very high accuracy, and accordingly, is very weak against external vibrations or impacts to be applied to the apparatus on the whole.
By way of example, on the assumption that the acceleration of the external vibrations is 9.8 m/sec.sup.2, the entire apparatus is to be displaced as much as .+-.2.5 mm in the case of a sinusoidal wave at 10 Hz. Accordingly, in order that the apparatus functions at high reliability with respect to external vibrations of the acceleration of 9.8 m/sec.sup.2 at 10 Hz, loop gain of 74 dB at 10 Hz from the focus control system and that of 88 dB at 10 Hz for the tracking control system, are at least required. Moreover, high speed response is also required for the transfer control.
However, there are further the vertical deviation acceleration of the recording medium with respect to the focus control, and the radial deviation of the track with respect to the tracking control. Accordingly, the loop gains for the above focus control system and tracking control system are still required more.
In order to constitute the control system having such high gains as described above, it is indispensable to detect the control signal at a high S/N ratio, and to provide a control means with a high performance. In the conventional arrangements, however, since the above conditions are not satisfied, they are not strong enough against vibrations or impacts applied from outside, with a great possibility of track jump, defocusing, etc. For example, if the track jump takes place during recording of a signal, not only the signal itself is not correctly recording, but there are such drawbacks in the worst case that other tracks are damaged or signals already recorded on other tracks are undersirably spoiled, etc. Meanwhile, in the case where track jump takes place during reproduction of already recorded signals, it is needless to say that the expected signal can not be properly reproduced.
As described so far, the conventional apparatuses of this kind are very low in the reliability against vibrations or impacts to be applied from outside.